In medical practice, meanwhile, various tests and diagnoses are performed by imaging the affected area within the internal organ, skeleton or the like by means of, for example, ultrasonic waves or X-rays. This enables to visually recognize the information of organs or the like of a body, and it is expected to enhance diagnostic accuracy.
Meanwhile, in recent years, there are growing demands for quantitative or automatic diagnosis aided by a computer, and a computer-aided diagnosis (CAD) system using a computer is studied vigorously. In order to perform CAD using a computer, it is important to precisely extract the information necessary for diagnosis, that is, the region of an organ or the like from the image obtained by imaging an affected area.
As the technique of extracting a contour of an object from an image, the technique using an active contour model such as a level set or snakes is known. The active contour model refers to the technique of extracting a contour of an object in an active manner from a curve (active curve) that iterates deformation in accordance with a predetermined deformation mode. The deformation mode of an active curve is defined by the “energy of an active curve” that is a value obtained by quantitatively evaluating the state of a curve. The “energy of an active curve” is predefined so as to be minimized at the time when the active curve extracts a contour of an object. Therefore, a contour of an object can be extracted by deforming the active curve such that the energy of the active curve is minimized and finding the stable state in which the energy is minimized
That is, the active contour model is considered to be the technique of extracting a contour of an object by defining an energy function that is minimized when the active curve extracts a contour of an object and finding the stable state in which this energy function is minimized. The process of minimizing an energy function is generally performed by an iterative computation.
The energy of an active curve is expressed by, for example, the function E of (Expression 1) below.E=Wi*Ei+We*Ee  (Expression 1)
That is, the energy function E is defined as a weighted linear sum of a plurality of kinds of energy terms defined correspondingly to the state of the active curve. For example, the energy term (internal energy term) Ei defined from the shape of an active curve itself (for example, smoothness) or the energy term (external energy term) Ee defined from the degree of coincidence between an active curve and a contour of an object can be employed as the energy term. As described above, however, the energy function E needs to be defined to be minimized when the active curve extracts a contour of an object in an appropriate manner, and accordingly each energy term needs to be formulated so as to have a smaller value as the active curve approaches a target shape.
For example, the external energy term Ee is formulated so as to have a smaller value as the degree of coincidence between an active curve and a contour of an object becomes higher. The introduction of the external energy term Ee into the energy function E allows an active curve to be distorted in such a manner of faithfully extracting a contour of an object. However, if the energy function E is defined by the external energy term Ee only, the extracted contour faithfully reproduces noise or the like appearing in an image as well, which makes it difficult to recognize the shape of an object. Therefore, the internal energy term Ei that is formulated so as to have a smaller value as the active curve becomes smoother is introduced into the energy function E. The introduction of the internal energy term Ei into the energy function E allows an active curve to be distorted in such a manner of making the shape of the active curve itself smooth. Two terms of the internal energy term Ei and the external energy term Ee are introduced into the energy function E as described above, and those are balanced in an appropriate manner, whereby it is possible to extract the contour of an object in an appropriate shape.
Note that how to balance a plurality of kinds of energy terms Ei and Ee in the energy function E depends on, for example, an image state, an object shape, smoothness of an object, which is not determined generally. Then, weighting factors Wi and We are added to the energy terms Ei and Ee, respectively, and the values thereof are adjusted in accordance with the object shape or the like, thereby adjusting the balance between the respective energy terms Ei and Ee. For example, an increased weighting factor Wi of the internal energy term Ei allows the active curve to have a smoother shape at the time when the energy function E is minimized. On the other hand, an increased weighting factor We of the external energy term Ee allows the active curve to capture a contour of an object more faithfully at the time when the energy function E is minimized. In this manner, the state of the curve to be obtained varies depending on a value of a weighting factor.
For example, Patent Document 1 and Non-Patent Document 1 describe the outline of snakes that is one of the representative active contour models.